U.S. Pat. No. 3,393,634 to Blackford describes a method and apparatus for loosening fibers and wood chips. For purposes of contrasting his process, Blackford describes a general method for kraft pulping in column 1, lines 12-22, inclusively. Blackford goes on to state that feed chips in any given batch vary considerably as to size, especially the length and larger transverse dimensions. And, because of the dimensional differences between respective chips, overcooking or undercooking of the chips, or, alternatively, longer cooking times and lower pulp yields, will result. A further problem resides in the presence of compression and knot wood which require even longer cooking times in order to achieve the requisite degree of pulping. Therefore, these latter materials generally need recycling through the pulping system several additional times in order to achieve the desired pulp quality. This is, of course, a distinct economic problem to the entire pulp and paper industry and is even more acute in older, recovery-limited mills where capacity is at a premium.
Mechanical methods have been employed in reducing the gross size of the chips fed to the pulping operation. The prior art suggests, however, that any significant amount of additional chipping of the feed chip stream causes excess damage to the cellulosic fiber itself, which in turn reduces the strength properties of the paper produced therefrom to a point below specified minimums.
In his patent, Blackford provides an apparatus which compresses the feed chip stream to a fraction of its original thickness without damaging, to any substantial degree, the fibers which form the chip structure. This means of compression of the chips loosens the fibers and renders the chips more porous and accessible to pulping liquor penetration. The apparatus employed by Blackford is shown in FIGS. 1-4, and described in detail in column 2, lines 10-48.
The Blackford process, as its overall objectives, provides a method and apparatus for controled compression of the above described chips to promote delamination thereof. For purposes of the present invention, delamination is defined as "compression by cleavage in a plane parallel to the fibers so that they are not substantially damaged by the mechanical forces imparted to them." By loosening and exposing the fibers, Blackford states, through compressive delamination, "the cooking liquid can penetrate the chips and affect the lignin uniformly throughout the chips irrespective of their size (emphasis added), thereby obtaining faster cooking, more uniform pulp, no uncooked shives, increasing the yield of pulp, and to render the moisture content of the chips more uniform." Therefore, the pulping yield will depend to a great extent on the degree to which the chips are delaminated, especially the troublesome knot wood and compression-wood chips.
Although the above objectives are accomplished to a certain extent by Blackford, several problems are present, however, when the above process is employed. Because of the variation in the size of the chips in the feed stream, a given opening formed between the press roll, i.e., the nip, cannot efficiently and effectively compress and delaminate the total size spectrum of entering chips. This problem results from the critical relationship required between the nip size and the thickness of the chips in order to effectuate the requisite degree of delamination. Blackford states that the space between the rolls should be approximately one-hundredth to five-hundredth inch. In addition, he goes on to say that the space between the rolls is small enough to compress the chips "to at least approximately one-fifth of their original thickness but not more than approximately one-tenth of their original thickness . . . ." Accordingly, if the space between the rolls is calibrated to compress the larger chip fraction of the total stream, i.e., chips having a thickness greater than one-half inch, a substantial amount of the smaller sized fraction, which represents a majority of the chip stream, including compression wood and knots, will pass through the nip without being effectively delaminated. As will be evident from the data hereinafter presented, the above smaller sized chip portion represents about 86% of the total chip feed stream. On the other hand, if the nip is set for compression of the majority fraction, i.e., chips having a thickness of less than one-fourth inch, a substantial amount of the larger chip fraction will suffer mechanical damage and/or, if the resistance to deformation of the chips exceeds the frictional forces attempting to draw them through the nip, will slide back and forth between the rolls until they are physically removed. Moreover, while the large chips are stymied, other smaller chips will drop through the nip opening uncrushed, or, alternatively, will pile up behind the stagnant chip. This, of course, will cause an interruption in the continuous operation of the process, thereby reducing the efficiency of delamination of the system. More specifically, the efficiency of delamination is a measure of the through-put, in tons per day per linear foot of machine width, of the chips compressed by a given process. Preferably, the through-put will be at least 75 tons per day, and more preferably at least 25 tons per day, per linear foot of roll press width. In any case, the overall yield of product, based on the efficiency of delamination, final product pulp conversion, and physical properties of the sheets produced therefrom, respectively, is substantially lower for the prior art processes than for the process of the present invention.
Colombo et al., in Canadian Pat. No. 677,418, and in an article entitled "Effects of Mechanical Chips Treatment on Pulp and Paper for Kraft Cooking of Softwood" in Svensk Papperstidning, Volume 63, No. 15, pages 457-471 (August 1960), also recognize the need for controlling clearance and pressure between the cylinders of an apparatus similar to that employed in the Blackford patent. However, Colombo et al. further provide, in order to minimize mechanical damage, that the chips be moved through a series of press rolls, of decreasing clearances, in which the chips are successively subjected to decreasing amounts of radial compressive forces, well below their respective elastic limit. Minimizing initial chipping operation is also suggested by Colombo. This is accomplished by adjusting the chippers to produce big chips and reducing the small sized fraction. The process also includes the separation of the feed chip stream into respective large and small fractions prior to employing successive compression steps. Because of this initial separation step, the feed stream contains a significantly greater proportion of large sized chips than in either Blackford or in the process of this invention. Chip fraction and chip thickness distribution data, with respect to the process of this invention, will be hereinafter set forth to illustrate this difference. Moreover, it is estimated that a series of about six to 10 pairs of cylindrical roll presses would be required to sequentially delaminate initial chip feed, step-by-step, in preparation for pulping. Therefore, in this roll press series, a greater potential exists for any of the problems present in the Blackford process to interrupt and materially affect the compression sequence of Colombo. This, indeed, is a significant problem since the Colombo process is quite complex to operate and exceedingly costly to install and maintain.
U.S. Pat. No. 3,070,318 to Blanchard provides a system for removing bark from chips in which the chips are first debarked and compressed simultaneously between a pair of rolls 1 and 2, roll 2 being knurled. The material exiting the rolls is then separated into two fractions by an inclined screen. The smaller fraction passing through the screen is again compressed and debarked using a pair of rolls similar to the aforementioned rolls 1 and 2. The compressed chips exiting the second pair of rolls is then recombined with the larger fraction which did not pass through the screen and the composite chip stream is compressed and debarked employing a third pair of press rolls. The latter pair of rolls has a tooth-in-groove peripheral configuration. The process of the Blanchard patent suffers from the above drawbacks outlined with respect to both the Blackford and Colombo processes.